1. Field of the invention
The present invention relates to a gas burner,of the cold nozzle type, for causing high speed external combustion of a gas mixture.
It applies more particularly but not exclusively to the low temperature heating of plastic material films, for example polyethylene films to cause retraction thereof, by means of a gas flow coming from the combustion of a combustible gas such as propane and air.
To comply with this type of application, the burner must be designed so as to produce a gas flow having a temperature of the order of 120.degree. to 540.degree. C. at a predetermined distance from the burner (distance at which the plastic film to be treated must be disposed).
At this distance, the temperature must be relatively homogeneous and the gas flow free of combustion material, if any possibility of burning, scorching and blistering of the film is to be avoided.
2. Description of the Prior Art
In order to reach this result, a burner has already been proposed including an injection device adapted for producing a high speed flow of a fuel gas mixture and injecting this flow into a tubular shaped burner head comprising successively:
a pressure recovery chamber having, in the plane of symmetry of the head, a divergent shape and inside which the gas mixture coming from the injection device develops in a fan configuration,
an ignition chamber of substantially constant and rectangular section,
two deflectors which extend respectively the two large sides of the ignition chamber and which converge towards each other, these two deflectors having two rectilinear front edges forming therebetween a passage of reduced width, and
a diffusion grid of substantially semi-cylindrical shape fixed inside the head at the level of the junction of the two chambers, this grid forming a convex dividing wall, directed parallel to said edges in said plane of symmetry, and whose concavity is oriented inwardly of the pressure recovery chamber.
With the above described structure, the burner head is not licked by the flame and therefore is not subjected to appreciable overheating.
However, contrary to what was expected, the flame produced does not have a strictly rectilinear and homogeneous leading edge.
In fact, it has been found in use that the leading edge of this flame in its central region has a substantially rectilinear edge bordered laterally by two respective horn-shaped protuberances.
It is clear that the presence of these side horns means that the advantages offered in other respects by this type of burner cannot be fully used.
A first aim of the invention is therefore to overcome this drawback. Moreover, it so happens that the burners of this kind are very widely used as "hand burners" and consequently have a handle which the operator holds in his hand.
During his work, he must sweep over the film to be treated holding the burner turned so as to keep the rectilinear front edge of the flame parallel to the zone of the film undergoing treatment, in order to obtain homogenous heating of this zone.
Now, such parallelism is difficult to acquire then to maintain.
Consequently, it is difficult to avoid untimely orientations which cause temperature heterogeneities which risk deteriorating the film.
The invention also overcomes this drawback.